1. Field of the Invention
The present invention relates to a 4,4′-Bis(carbazol-9-yl)-biphenyl (CBP) based silicone compound and an organic electroluminescent device using the same. More particularly, the present invention relates to a CBP based compound that may be used as a host material for various phosphorescent or fluorescent dopants emitting red, green, blue, or white light, and an organic electroluminescent device using the CBP based compound which has enhancements such as, for example, high efficiency, high luminance, long life span, and low power consumption.
2. Description of the Related Art
Electroluminescent devices (EL devices) are self emission type display devices that have advantages such as a wide viewing angle, superior contrast, and fast response speed.
The EL devices are classified into inorganic EL devices and organic EL devices according to the material utilized in a light-emitting layer. The organic EL devices have advantages over the inorganic EL devices, such as high luminance, low driving voltage, fast response speed, and multi-coloration.
Generally, the organic EL devices have a sequentially stacked structure of an anode, a hole transport layer, a light-emitting layer, an electron transport layer, and a cathode on a substrate. The hole transport layer, the light-emitting layer, and the electron transport layer are organic films comprising an organic compound.
The organic EL devices having the above-described structure are driven in accordance with the following principle.
When a voltage is applied to the anode and the cathode, holes from the anode migrate toward the light-emitting layer via the hole transport layer. On the other hand, electrons from the cathode are injected into the light-emitting layer via the electron transport layer. Thereafter, the electrons and the holes recombine with each other at the light-emitting layer to generate excitons. When the excitons are converted from an excited state to a ground state, a fluorescent molecule of the light-emitting layer emits light, which displays an image. Here, light emission through conversion from a singlet excited state to a ground state is called “fluorescence”, and light emission through conversion from a triplet excited state to a ground state is called “phosphorescence.” With respect to fluorescence, the proportion of singlet excited state is 25% (the proportion of triplet excited state is 75%), and thus, there is a limitation on light emission efficiency. On the other hand, with respect to phosphorescence, the triplet excited state and the singlet excited state may be used. Therefore, a theoretical internal quantum efficiency may reach 100%.
By way of an example of an organic electroluminescent device using phosphorescence, a highly efficient, green and red-emitting organic electroluminescent device may use a CBP based host, and a phosphorescent dopant such as Ir(ppy)3 (ppy is phenylpyridine) and PtOEP (platinum(II) octaethylporphyrin) having heavy elements with significant spin-orbit coupling such as Ir and Pt in the center thereof may be utilized. However, the organic electroluminescent device has a short life span of 150 hours or less since CBP has a low glass transition temperature of less than 110° C. and is easily crystallized, which makes it difficult to provide a commercially suitable product.
As another example of an organic electroluminescent device using phosphorescence, an organic electroluminescent device uses a blue phosphorescent dopant (4,6-F2ppy)2Irpic with a fluorinated ppy ligand structure. The energy band gap between the triplet state and the ground state of CBP is high enough to provide an energy transition for green and red phosphorescent dopants, but is smaller than the energy band gap of a blue phosphorescent dopant. Therefore, it is reported that a very inefficient endothermic energy transition, not an exothermic energy transition, occurs even when a material such as (4,6-F2ppy)2Irpic with photoluminescent (PL) peaks at 475 nm and 495 nm is used. For this reason, the CBP based host cannot provide a sufficient energy transition for a blue phosphorescent dopant, thus causing problems such as low-efficiency blue light emission and a short life span.
In addition, U.S. patent application Laid-Open Publication No. 2002/0125818 A1 discloses an organic electroluminescent device using a CBP based compound.
Recently, a mCP (1,3-Bis(carbazol-9-yl)-benzene) compound having a triplet energy band gap higher than CBP has been used. However, the mCP compound has problems such as a molecular weight that is too small and a low stability. In this regard, to obtain high-efficiency blue light emission characteristics exhibiting a long lifetime, it is very beneficial to develop a host material with a triplet energy band gap that is larger than the triplet energy band gap of CBP for efficient energy transition for a blue-emitting dopant and which has a high glass transition temperature (Tg).